Silicon carbide (SiC) is widely considered as the power semiconductor of choice due to its wide energy band-gap, high breakdown field strength, high operating junction temperature, high carrier saturation drift velocity, and high thermal conductivity. SiC devices offer great potential for use in high-temperature, high-power, high-frequency, and radiation-resistant military applications such as ultra-compact, ultra-light power conversion units for in-ground power sources, hybrid military vehicles, electric aircraft and ships, and nuclear power instrumentation.
SiC devices can also be used in a wide range of commercial applications including hybrid electric cars, power supplies, and electric utility power units. SiC has several advantages over other competing wide band-gap semiconductors, such as, gallium nitride (GaN) and diamond, including commercial availability of substrate materials and known device processing techniques.
Twelve hundred volt/twenty amperes (1200V/20 A) SiC Schottky diodes are now commercially available from Cree, Inc., Durham, N.C. 27703. A 2000V SiC power Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) with a low specific on-resistance of 10.3 mΩ-cm2 was also reported recently. The latest breakthrough in defect-free SiC crystal growth technology from Denso Corporation, headquartered in Kariya, Aichi prefecture, Japan, is expected to expedite the large scale commercialization of SiC in the near future.
SiC devices can safely operate at a junction temperature up to 500-600° C. Such a high operating temperature range can substantially reduce or totally eliminate the need of bulky and costly thermal management components such as liquid cooling components, fans, and heat sinks commonly used in silicon-based power electronic systems, leading to smaller, lighter, more efficient, and more reliable power converter design particularly suitable for military applications.
The thermal management subsystems in many silicon-based power converters account for up to 30-40% of the total system cost. The cost benefit at the system level resulted from higher operating temperatures can partially offset the high cost of today's SiC components, eventually leading to a net cost reduction as the SiC technology evolves. However, a major limitation to fully realizing the potential of SiC and other wide band-gap semiconductor materials is the lack of qualified high-temperature packaging systems, particularly those with high-current and high-voltage capabilities required for power conversion applications. The commercial SiC Schottky diodes currently available are only rated for an operating junction temperature up to 175° C. due to the limitation of the conventional plastic packages used.
The overall goal of packaging is to distribute signal and power, dissipate heat, protect the devices enclosed, and ensure reliable operation of semiconductor devices. Considerable progress has been made in the field of high-temperature electronics and electronic packaging in the past two decades as discussed below in the description of metal, ceramic and plastic packages. The high-reliability packaging schemes used today for military applications do offer considerable high temperature operation capability, but fall short on power handling capability and other requirements.
Metal packages for semiconductors are commercially available and can be a single metal package in a circular shape, a single metal package in a rectangular shape or, a single metal package with glass-sealed output leads. A single metal package is usually made of gold-plated Kovar (an alloy of 53% iron, 29% nickel). The output leads are sealed into the Kovar package sidewalls or floor using glass-to-metal seals or ceramic feedthroughs. The metal packages are designed with welded metal lids. Gold (Au) wires are typically used to interconnect the semiconductor chip and the metal leads using a wirebonding technique.
Some metal packages have been evaluated to 400° C. with satisfactory results, as reported in the National Research Council, Materials for High Temperature Semiconductor Devices, National Academy Press, 1995. However, these packages are limited to relatively low current applications due to the use of thin Au wires (typically 2-5 mils in diameter). Paralleling these packages for higher current capability may be feasible for low-frequency power converters, but does not provide a viable solution for high-density high-performance power electronic systems due to the inevitable large parasitic impedance and physical dimensions associated.
Ceramic packages are another widely used type of package that provides good high-temperature performance. Ceramic packages are manufactured using a co-fired tape process and have an advantage over metal packages because they can avoid the use of expensive fragile glass-to-metal seals. The packages can be sealed either by soldering or welding. Temperature limitations for ceramic packages depend on the type of sealing method used. However, ceramic packages are not suitable for high-current applications due to the high resistance of the refractory metal interconnects used inside the packages.
A third type of prior art packaging is the plastic package. Plastic packages are the most common type of electronic packages for conventional temperature electronics. A semiconductor chip is attached to a metal leadframe (usually formed on a stamped copper sheet). Wire bonding is then performed to interconnect the chip's top surface to the metal leadframe. Subsequently, thermosetting plastics such as alkyds, allyls, epoxies, phenolic, unsaturated polyesters, polyimides, polyurethanes, and silicones, can be injected, compressed, or transferred into the mold with the metal leadframe as the insert.
Alternatively, thermoplastic plastics such as acrylic, fluoropolymers, liquid crystal polymers, and nylons can be also used as the encapsulation molding compound. Plastic electronic packages offer low cost, high manufacturability, and compact package sizes. However, today's plastic packages are not suitable for operating temperature above 150° C. For example, the glass transition temperature of commonly used packaging epoxies is in the range of 130-170° C., which limits the operating temperature range of the package.
Plastic packages also subject wire bonds to extreme stresses if the package undergoes large temperature swings. Nevertheless, the continued use of plastic package in the high temperature ranges is highly desirable. Recent advances in high temperature polymer materials have shown great promise. For example, Quantum Leap Packaging, Inc. (Wilmington, Mass.) has developed a new Liquid Crystal Polymer (LCP) as an encapsulation material to compete with the more costly and traditional ceramic and metal component packaging. A key feature of Quantum Leap's LCP material is that it supports up to 400° C. applications while still offering MIL-SPEC hermetic properties. Other high temperature injection molding plastics have also been reported in High Temperature Electronics, edited by R. Kirschman, IEEE Press, 1999.
A fourth type of conventional hybrid power module exists. For conventional temperature high power electronic systems, multi-chip hybrid power modules are predominantly used. The hybrid modules distribute signal and power, dissipate heat, protect the devices enclosed, and serve as the basic power electronics building block (PEBB).
The state-of-the-art power module technology, initially developed in mid-1980's, mainly relies on the use of aluminum wirebonds, direct-bond-copper (DBC) ceramic substrates, and copper base plates, as shown in FIGS. 1A and 1B.
FIG. 1A shows the basic structure of a hybrid power module in a case 5 using aluminum wirebond 10 with a semiconductor chip 15 on a copper layer 17 supported by a ceramic DBC substrate 19 and Cu base plate 20 wherein the aluminum wirebond 10 is connected to copper terminals 25, 26. FIG. 2B is a photograph of a half-bridge Si-IGBT/SiC-Diode power module from Cree/Powerex. The SiC Schottky diodes 27 and Si-IGBTs 29 rely on the use of aluminum wire bonds, direct-bond-copper ceramic substrates and copper substrates in the arrangement shown.
The thin aluminum wirebonds suffer from high parasitic impedance, fatigue-induced lift-off failures, and inability of removing heat. The DBC ceramic substrate, aluminum oxide (Al2O3) or more expensive aluminum nitride (AlN), provides electrical isolation but inadvertently increases the package thermal resistance. The thick copper (Cu) base plate serves as heat spreader but considerably increases the weight, size, and thermal resistance of the power module. The easy oxidation of Cu, low melting temperatures of the tin/lead (Sn/Pb) solder joints of chip-to-DBC and DBC-to-baseplate, and low glass transition temperature of the epoxy case all prevent the use of the conventional power modules in high temperature applications.
Most recently, within the last five years, federal agencies including the Department of Defense (DOD) and the National Aeronautical Space Administration (NASA) have funded several Small Business Innovation Research (SBIR) projects to develop high-temperature electronic packages. Such efforts have led to the development of high temperature power modules based on aluminum nitride (AlN) ceramic substrates. AlN ceramic has excellent high temperature stability, high thermal conductivity and a coefficient of thermal expansion (CTE) closely matching that of SiC and Si materials.
One approach is to use a molded AlN package with nickel (Ni) conductor traces actively brazed to the AlN substrate. Semiconductor chips are soldered directly to the Ni conductors and aluminum wirebonds connect the devices to interconnect circuitry on the AlN substrate. This approach is very similar to the conventional ceramic hybrid packaging technology. Another slightly different approach is to use a flat AlN substrate with patterned metal films, and a Kovar alloy case and Kovar feedthroughs brazed to the AlN substrate. Yet another approach was to use direct die attaching and flip-chip techniques in addition to the use of AlN substrates, as explained by in an article found on the Internet by T. O. Martin and T. R. Bloom, “High Temperature Aluminum Nitride Packaging,” (http://www.ctscorp.com/techpapers/techpapers.html). This approach eliminates the need for wirebonds which are known to be the weakest link at high temperatures. However, this approach does not offer high current handling capability due to the high resistance of the thick-film Au or Pt interconnection layer used.
Problems still exists with the prior art described above. Many factors have to be considered in the preparation of satisfactory high-temperature electronic packaging, so the devices are often very complicated and costly. For example, some of the considerations, include, but are not limited to, characterizing materials and their interactions at high temperatures, minimizing mechanical stresses caused by thermal expansion mismatches, providing a suitable path for heat dissipation, providing environmental protection, facilitating system integration (from chip-level to board-level). In addition, high current capability and low package interconnect impedance are critical factors in the design of hybrid power modules for power electronic systems.
None of the prior art references is entirely without technical merits, those aluminum nitride (AlN)-based high temperature modules are essentially very costly duplicates of the existing conventional temperature hybrid module technology, which is, unfortunately, a 20-year old technology due for an overhaul itself. The biggest problem of a conventional power module is the complexity of its material system, which is comprised of multiple semiconductor chips, one or more ceramic substrates with patterned metal interconnect films, a metal baseplate, Al or Au wirebonds connecting the semiconductor chips to the substrate metal layer, soldering joints of semiconductor-to-substrate and substrate-to-baseplate, epoxy sidewall module case, external metal posts or feedthroughs, and silicon-gel potting material. Such a module construction method unnecessarily increases the module's weight, size, and junction-to-case thermal resistance.
More critically, the complex material system creates many joints and interfaces between dissimilar materials, which tend to cause reliability concerns at high temperatures or under thermal cycling conditions. For example, it is well known that aluminum wirebonds tend to fail by creep deformation at high temperatures, and tin/lead (Sn/Pb) solder joints suffer from voids or delineation under thermal cycling in the conventional power modules. While the reliability issue can be mitigated to a certain extent through the selection of new materials and processes (often with the penalty of a sharp increase in cost) as suggested by the aforementioned high temperature modules, it would be much more advantageous to have a simplified material system to begin with.
Furthermore, in the conventional hybrid power modules (and their high temperature variants alike), heat is solely dissipated from the semiconductor chips downward to the ceramic substrate, metal baseplate, and eventually to the external thermal management components such as the heat sinks or liquid-cooling cold plates. Note that the top side of the power module does not contribute at all to removing heat from the semiconductor chips. While this may be tolerated in the conventional temperature power converters equipped with sophisticated cooling subsystems, it is highly desirable to dissipate heat from both sides of the module in high temperature electronic systems, where we expect to employ very simple or no cooling design at all.
In short, the existing hybrid power module technology, limited by its material and construction complexity, does not provide a good platform for new high temperature module development. It is simply not enough just to substitute the materials in the conventional module architecture with newer and more costly high-temperature alternatives. More disruptive technological innovation in both module architecture and material selection is needed to provide smaller, lighter, cheaper, and more reliable modules for high temperature power electronics in both military and commercial applications.